The best use of Ravens

I found that huge and dangerous Zerg units like Ultralisks, Broodlors and some smoller of the Heroic Monstres are concentrating for the future ONE HUGE DEADLY WAVE arround hatheries.So this is advisable that part of players would concentrate not on the building the Saving Ships and Saving Bacons (and undergo the extra atacks this way ), but on making many Ravens and going arround the map and bit dangerous guys from the air and from the safe distance, away from own defencive positions. But someguys still need to build bigger defence and buld Saving Ships (atracting the monsters)